Under the recent trend in the art to making electronic elements and parts in a thin geometry and in light weighing, so-called "bear-chip mounting" and "flip-chip bonding" for directly mounting IC chips on electronic elements and parts have found applications to the assemblage of such electronic elements and parts.
For direct assemblage of IC chips on substrate of circuit pattern, there have hitherto been practised, for example, 1) the "wire-bonding" technique in which the terminal electrodes of the IC chip are connected using gold microwires with corresponding terminals in the circuit pattern formed on the substrate, 2) the "face-down" technique in which the terminal electrodes of the IC chip and the corresponding terminals in the circuit pattern formed on the substrate are connected by means of "reflowing" of solder and 3) the anisotropic connection technique in which the terminal electrodes in a form of microbumps on the IC chip and the corresponding terminals in the circuit pattern formed on the substrate are connected by press-fixation of the IC chip and the circuit pattern-carrying substrate together under interposition of a layer of an anisotropically electroconductive adhesive therebetween. Among them, the anisotropic connection technique using an electroconductive adhesive (including an anisotropically electroconductive adhesive film prepared by forming a layer of an anisotropically electroconductive adhesive on an exfoliative film, namely, a film easily peeled off therefrom, with subsequent heating and drying of the layer) has recently attained wide uses, since it provides many profits, such as convenience in the practical operation, high reliability of establishment of electric connection and unnecessity of sealing of the assembly with simultaneous permission of a high density assembly at a minimum cost.
In accompaniment with the decrease in the size of the IC bumps (projecting terminal electrodes) and with the trend to designing the circuit pattern in a finer pitch, however, it becomes necessary to use electroconductive particles of smaller particle size in the anisotropically electroconductive adhesive and, in addition, there is a common trend to the use of higher content of the electroconductive particles in the adhesive for increasing the reliability of establishment of an assured electric connection.
Under these circumstances, there occurred problems in that fluctuation of condition for building up the electric connection by the adhesive is brought about due to the secondary agglomeration of the conductor particles and short-circuiting between electric lines in the circuit pattern on the substrate may occur, when electroconductive particles of smaller particle size is employed, and in that short-circuiting between electric lines in the circuit pattern on the substrate may also happen, when the content of the electroconductive particles is increased.
As countermeasures therefor, attempts have been proposed, such as those in which insulator-coated conductor particles having each particle coated with a layer of insulator is used and in which a plurality of anisotropically electroconductive films are used under lamination in order to prevent dislodging or exilation of the conductor particles away from the region of the terminal electrode during the course of building up of the electric connection by the press-fixation.
Use of such insulator-coated conductor particles may give birth to some fear of reduction in the long term reliability for assured electric connection due to the hardness and elasticity thereof. When the blending proportion of the insulator-coated conductor particles, which are constituted, in general, mainly of a particulate product having an average particle size of about 5 .mu.m, is increased to a value of, for example, 40,000 particles/mm.sup.2 -film, a reliable insulation or exclusion of occurrence of short-circuiting may difficultly be maintained for binding an IC chip of a bump pitch of 10 .mu.m or less.
While, on the other hand, use of multilayered anisotropically electroconductive film may permit increase in the blending proportion of the conductor particles up to, for example, about 80,000 particles/mm.sup.2 -film for smaller size conductor particles having an average particle size of about 3 .mu.m, it is necessary in this case to form bumps in a higher accuracy with simultaneous requirement of rigorous administration of precision of the pressing condition upon establishment of the electric connection, resulting in a high production cost.
In Japanese Patent Application Kokai Hei 4-174980 A, an electrically connecting composition for connecting lines of electric circuit is disclosed, which comprises, on the one hand, an insulator-coated particulate product in which each particle of an electric conductor deformable upon being heated is coated with a layer of a thermoplastic insulator and, on the other hand, a particulate thickness-regulating product having a hardness higher than that of the insulator-coated particulate product, wherein both particualte products are dispersed in a matrix of an adhesive material of insulating nature capable of exhibiting a plastic flow when heated.
This electrically connecting composition is, however, difficult to acquire a high reliability for establishing an electric connection when the thickness-regulating product is made of an electric insulator, since this product does not participate in the establishment of an electric connection. When the thickness-regulating product is made of an electric conductor, a higher blending proportion thereof may permit occurrence of short-circuiting, obstructing thus the reliability of insulation or exclusion of occurrence of short-circuiting. Moreover, when the thickness-regulating product consists of particles having uneven particle sizes, the thickness of the resulting assembly is determined by the largermost particle in this product, as the thickness-regulating particles have high hardness, so that particles having sizes smaller than the largermost one cannot participate in the establishment of an electric connection, resulting also in a defective reliability in the establishment of electric connection.
In Japanese Patent Application Kokai Hei 9-102661 A, a method for providing electric connection between terminal electrodes using an electroconductive microparticulate product having a certain compressive hardness (K-value) and a specific restorability from deformation.
Use of such an electroconductive micro-particulate product as above may, however, difficultly result in attainment of high reliabilities both in the establishment of electric connection and in the insulation upon connection of an IC chip of a fine pitch circuit pattern.